Enter your beam's span, width, and depth to instantly calculate concrete volume in cubic yards, total weight, and rebar requirements — for rectangular and T-beams.
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Formulas aligned with ACI 318
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✓ Rectangular & T-beam types✓ Volume, weight & rebar✓ Works on any device✓ Last verified May 2026
Center-to-center distance between supports.Please enter a valid span greater than 0.
Web width (stem width for T-beams). Typical: 10–18 in.Please enter a valid width greater than 0.
Overall height of the beam from bottom to top. Includes flange for T-beams.Please enter a valid depth greater than 0.
Add 5–10% for standard pours. Add 10–15% for complex forms.
Total width of the top flange (slab overhang both sides + web). See ACI 318-19 §6.3.2.Please enter a valid flange width greater than 0.
Thickness of the top flange (usually the slab thickness). Typically 4–8 in.Please enter a valid flange thickness greater than 0.
$
Leave blank to skip cost estimate. US average: $110–$160/yd³ for structural ready-mix.
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Your Beam Concrete Estimate
Concrete Volume (with waste)
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Cubic Yards (yd³)
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Cubic Feet (ft³)
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Cubic Meters (m³)
Beam Weight & Cross-Section
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Weight (lb)
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Weight (US tons)
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Cross-Section (in²)
—Beam Span
—Beam Type
—Net Vol (no waste)
—Waste Factor
Estimated Material Cost
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Concrete material cost only. Add form labor, reinforcing steel, shoring, and finishing for a full project budget. Use our Full Project Estimator for a complete breakdown.
T-BEAM (additional steps):
Step 1a: Flange area (ft²) = Flange Width (ft) × Flange Thickness (ft)
Step 1b: Web area (ft²) = Web Width (ft) × (Total Depth − Flange Thickness) (ft)
Step 1c: Net cross-section = Flange area + Web area
Step 2–4: Same as rectangular beam
Select your beam type.
Choose Rectangular for a simple prismatic beam. Choose T-Beam if the beam is cast monolithically with a slab above — this is the most common configuration in floor and roof systems. The T-beam option adds flange width and flange thickness fields that represent the slab portion above the web.
Enter span, width, and depth.
Span is the center-to-center distance between supports — not the overall beam length. Web width is the stem dimension (typically 10–18 inches for residential, 12–24 inches for commercial). Total depth is the full height from the bottom of the beam to the top of the slab for T-beams, or the bottom to top of the web for rectangular beams.
For T-beams, add flange dimensions.
The effective flange width is defined by ACI 318-19 §6.3.2 as the lesser of the center-to-center spacing of adjacent beams, span divided by 4, or web width plus 8× the flange thickness on each side. Ask your structural engineer for the design value. Flange thickness is typically the slab thickness — usually 4–8 inches.
Use the volume figure to order concrete.
The cubic yards result — including your waste factor — is what you communicate to the ready-mix supplier. For structural beams, always use the result with waste. The weight figure is useful for shoring design: your temporary support system must safely carry the wet concrete weight plus construction live loads before the beam gains strength.
⚠ Pro Tip: The single most common beam concrete error is under-sizing the form. Beam forms deflect under wet concrete pressure — especially on wide, deep beams. Always clamp or brace forms designed for beam pressures, not just slab pressures. Concrete at the bottom of a 24-inch deep beam exerts roughly three times the lateral pressure of a 4-inch slab. A blown form mid-pour is not recoverable.
Concrete Beam Volume Formula
The calculation uses the standard volumetric approach for prismatic members, consistent with ACI 318 and standard structural engineering practice. For rectangular beams, this is straightforward. For T-beams, the net cross-sectional area is computed by summing the flange area and the web-only area below the flange.
Step
Formula
Example: 20 ft span, 12 in wide, 24 in deep (Rectangular)
1. Convert dimensions to feet
Width: 12 in ÷ 12 = 1.0 ft; Depth: 24 in ÷ 12 = 2.0 ft
Width = 1.0 ft, Depth = 2.0 ft
2. Cross-section area
Width (ft) × Depth (ft)
1.0 × 2.0 = 2.0 ft²
3. Volume in cubic feet
Area × Span (ft)
2.0 × 20 = 40.0 ft³
4. Convert to cubic yards
ft³ ÷ 27
40.0 ÷ 27 = 1.481 yd³
5. Add waste factor (10%)
Volume × 1.10
1.481 × 1.10 = 1.629 yd³
6. Weight (no waste)
ft³ × 150 lb/ft³
40.0 × 150 = 6,000 lb = 3.0 US tons
Common Beam Size Reference Table
Concrete volumes for common rectangular beam sizes — 10% waste not included. Verify all dimensions against structural drawings before ordering.
Span
Width × Depth
Net Vol (ft³)
Net Vol (yd³)
Weight (lb)
10 ft
10 × 18 in
12.50 ft³
0.46 yd³
1,875 lb
16 ft
12 × 20 in
26.67 ft³
0.99 yd³
4,000 lb
20 ft
12 × 24 in
40.00 ft³
1.48 yd³
6,000 lb
24 ft
14 × 28 in
65.33 ft³
2.42 yd³
9,800 lb
30 ft
16 × 32 in
106.67 ft³
3.95 yd³
16,000 lb
40 ft
18 × 36 in
180.00 ft³
6.67 yd³
27,000 lb
50 ft
24 × 48 in
400.00 ft³
14.81 yd³
60,000 lb
Net volumes shown — no waste factor applied. Always add 10% minimum for ordering. All weights assume 150 lb/ft³ normal-weight concrete.
What Depth Does My Concrete Beam Need?
Beam depth is the primary driver of structural capacity. A deeper beam has significantly more moment of inertia and section modulus, meaning it resists bending far more efficiently than increasing width. The common rule of thumb for lightly reinforced simply-supported beams is span ÷ 12 for the total depth, but this is only a starting point — final sizing always requires structural engineering analysis per ACI 318.
Typical concrete beam depth guidelines by span and application. Engineer verification required before construction.
Span
Typical Depth (Simply Supported)
Typical Depth (Continuous)
Typical Width
Common Application
8–12 ft
10–14 in
8–12 in
8–12 in
Residential floor beam, header
14–18 ft
16–20 in
12–16 in
10–14 in
Commercial floor girder
20–24 ft
22–28 in
18–22 in
12–16 in
Parking structure, office floor
26–32 ft
30–38 in
24–30 in
14–20 in
Bridge approach, heavy warehouse
34–50 ft
42–60 in
32–48 in
18–30 in
Long-span roof, bridge girder
50 ft+
Engineer specified
Engineer specified
Engineer specified
Always requires PE design
Increasing depth is almost always more efficient than increasing width. Doubling the depth quadruples the moment of inertia; doubling the width only doubles it. If you're fighting deflection on a long span, go deeper before going wider — depth also reduces rebar requirements, which lowers the total project cost.
Common Mistakes When Estimating Beam Concrete
⚠️
Using overall beam length instead of clear span.
Concrete volume should be based on the actual pour length — from outside face of one support to outside face of the other. If your beam sits on 12-inch wide columns, the beam length is not the same as the column center-to-center span. Confusing these two figures leads to over-ordering by 5–15% on typical spans.
📐
Forgetting the flange volume in T-beams.
When a beam is cast monolithically with a slab, you cannot simply calculate web volume and ignore the flange. The flange is structural concrete — it must be included in the volume estimate. On typical floor systems, the flange can account for 30–60% of the total beam cross-section, so omitting it is a critical error.
🏋️
Ignoring wet concrete weight when designing shoring.
Normal-weight concrete weighs 150 lb/ft³. A 24-inch deep by 14-inch wide beam spanning 30 feet produces roughly 11,250 lb of wet concrete alone — before construction live loads. Temporary shoring that is designed for the finished beam load rather than the wet pour weight is a leading cause of formwork collapse incidents.
🔩
Not accounting for rebar displacement in volume-sensitive pours.
For most beams, rebar displaces 2–5% of the gross concrete volume. On small, heavily reinforced beams this becomes material. Rather than deducting rebar volume from the order, include it in your waste factor — the displaced concrete simply gets replaced by reinforcing steel and you don't risk running short.
📅
Scheduling beam pours without considering access and equipment.
Elevated beams require the concrete to be pumped or conveyed — you cannot simply chute from a truck. Coordinate pump truck access, boom reach, and form tie-down requirements before locking in a pour date. Structural ready-mix also often requires specific admixture packages (plasticizers, accelerators) that need to be specified at the time of ordering, not the morning of the pour.
Frequently Asked Questions
For a rectangular beam, multiply the beam's width by its depth by its span — all in the same unit — to get volume. Convert to cubic feet (if working in inches, divide by 1,728), then divide by 27 to get cubic yards. Add at least 10% for waste. For a T-beam, compute the flange area (flange width × flange thickness) and the web-only area (web width × web-only depth), add them together for the total cross-section, then multiply by the span.
A rectangular beam has a constant cross-section — the same width from bottom to top. A T-beam (or flanged beam) has a wider top portion (the flange) that is part of the connected slab, and a narrower stem (the web) extending below. T-beams are the default in most cast-in-place concrete floor systems because the slab and beam are poured together, creating a composite section. The flange significantly increases the compression zone area at the top of the beam, making T-beams far more efficient per pound of concrete than rectangular sections for positive moment regions.
Normal-weight concrete (the standard for structural beams) weighs approximately 150 lb per cubic foot, or about 4,050 lb per cubic yard. Multiply the beam's net volume in cubic feet by 150 to get the total weight in pounds. For example, a 20-foot span beam with a 12-inch × 24-inch cross-section has a volume of 40 cubic feet and weighs 6,000 lb. Lightweight concrete used in some floor systems weighs approximately 110–120 lb/ft³; always confirm the concrete unit weight on your structural drawings.
ACI 318-19 §6.3.2 defines effective flange width as the smallest of: (1) the beam span divided by 4, (2) the center-to-center spacing of adjacent beams, or (3) the web width plus 8× the flange thickness on each side (so web width + 16× flange thickness total). This limit exists because only the concrete immediately above the web efficiently participates in resisting bending — concrete farther from the web has reduced stress due to shear lag. Your structural engineer will confirm the controlling value. For volume estimation, use the full flange width shown on the structural drawings.
A commonly used rule of thumb for preliminary sizing is span/12 for the total depth of a simply-supported rectangular beam under uniform load, or span/18 for a continuous beam. These are starting points only — a 20-foot simply-supported beam might initially be sized at roughly 20 in depth (20 ft × 12 / 12 = 20 in), but the final size depends on applied loads, reinforcement ratio, deflection limits, and fire-resistance ratings. Never use rule-of-thumb depths as final design values; all structural concrete members must be designed by or reviewed by a licensed structural engineer.
Most structural concrete beams use 4,000 PSI (28 MPa) as a minimum per ACI 318. Heavily loaded transfer beams, long-span girders, and high-rise columns often specify 5,000–8,000 PSI for greater efficiency — higher-strength concrete allows smaller cross-sections. The structural engineer of record specifies the design strength (f'c) on the drawings. Never substitute a lower PSI mix on a structural project without written engineering approval — concrete strength directly affects the designed rebar layout and the member's capacity.
Rebar requirements must come from structural engineering calculations — this calculator provides volume only. That said, typical residential beams use 2–4 bottom bars of #5 or #6 rebar for longitudinal tension reinforcement. Stirrups (typically #3 or #4 bars bent into a U or closed loop) are spaced every 4–12 inches depending on shear demand and zone. The ACI minimum steel ratio for beams is approximately 0.33% of the gross area, but most practical designs land between 0.5–1.5% steel ratio. Use our Rebar Calculator to estimate quantities once you have the design drawings.
Technically yes, but it is rarely practical or advisable for structural applications. Bagged concrete (Quikrete, Sakrete) is typically rated at 4,000–5,000 PSI when properly mixed, which meets most structural minimums. The problems are consistency and volume. Mixing dozens or hundreds of bags by hand introduces variability in water-cement ratio, which directly affects strength. Most structural beams also require more than a practical hand-mixing volume — a single cubic yard requires about 45 bags of 60 lb concrete. For any structural application, ready-mix from a certified plant with certified batch tickets is strongly preferred and often required by the building inspector.
ACI 347 provides shoring duration guidelines, and local building codes govern the minimum. As a general rule, forms and shores supporting beams and slabs spanning more than 10 feet should remain in place for a minimum of 14 days at 50°F or warmer. For beams spanning more than 20 feet, 21 days is typical. Reshoring — a secondary system placed as original shores are removed — may be required on multi-story structures. Never strip shores based on calendar days alone without confirming concrete strength via cylinder breaks or maturity sensors, especially in cold weather.
ACI 318-19 Table 20.6.1.3 requires a minimum 1.5 inches of clear cover for beams not exposed to weather and cast in forms, assuming #5 rebar or smaller. For beams exposed to weather or soil, the minimum increases to 2 inches (#6 bar and larger) or 1.5 inches (#5 and smaller). Beams cast directly against and permanently exposed to soil require 3 inches. Cover protects rebar from corrosion and ensures adequate fire resistance. Always use plastic or concrete bar chairs — never wood scraps — to maintain consistent cover during the pour.
